The present invention is directed to an improved grass shear embodying a stationary toothed bed knife provided with cutting edges and a motor driven rotary blade cooperable therewith. The rotary blade member includes cutting edge means cooperatively engageable, in inclined scissor-like fashion, with the edges of the stationary bed knife teeth. The overall assembly is particularly adapted to be highly efficient, thus lending itself for use in a cordless, battery powered device. As used herein, the term "grass shear" describes a device intended to cut growth such as grass, weeds, shrubs and the like.
In my prior patent application, Ser. No. 413,533, filed Nov. 7, 1973, a similar grass shear is disclosed. This application is a continuation-in-part of the prior application which is incorporated by reference hereto in full. The grass shear disclosed, in my prior application, Ser. No. 413,533, in a preferred embodiment, includes a plurality of independent blade members carried by a motor driven hub assembly and resiliently biased in an axial direction by a spring towards a stationary toothed blade. The rotary blade members cooperate with the stationary teeth to produce scissor-like cutting action while permitting each blade to lift and slide over each stationary tooth independently of one another.
In accord with the present invention, the preferred embodiment consists of a rotating cutting blade member underlying and partially within a recessed portion of a rotating circular hub drive member. The rotating cutting blade member is positioned about a shaft which protrudes up from the center of the circular stationary bed knife. The rotating cutting blade member is held in a floating relationship above the stationary bed knife by washers and an E ring clip affixed to said shaft and the fact that blade fingers which are a part of the rotating cutting blade member are inclined downwardly so that lateral leading cutting edges formed on these blade fingers are in scissor-like engagement with lateral cutting edges formed on the bed knife teeth. Scissorlike engagement may also be achieved by inclining the bed knife teeth upwardly or by a combination of a lesser bend in both elements.
The rotating cutting blade member, being made of relatively thin hardened steel, is able to flex in springlike fashion, and as it is not affixed to it's driving member it is able to rock in see-saw like fashion about it's central axis as it rises while cutting on one tooth of the bed knife while at the same time the cutting edge formed on the opposite finger of the rotating cutting blade member is falling off of a trailing edge of a tooth on the opposite side of the stationary bed knife. Although, in the preferred embodiment, the rotating cutting blade flexes and thus provides it's own spring tension, separate spring means may be used and the rotating cutting blade may be of any desired thickness. For example, in such a configuration, the washers and the E ring clip which position the rotating cutting blade about the bed knife shaft may be eliminated and spring pressure applied upon a flat circular portion of the rotating cutting blade from above.
In the preferred embodiment the rotating cutting blade member consists of a flat central portion with a hole in it's center. Two blade fingers extend from this circular central portion, these fingers being substantially diametrically opposite to each other such as at a 12 o'clock and 6 o'clock position. At a 3 o'clock and a 9 o'clock position about this flat circular portion drive tabs protrude from the rotary cutting blade member and are bent 90 degrees upwards. These tabs fit into holes in the rotating hub drive member which will thus also rotate the cutting blade member. The two extending blade fingers are bowed downwardly, their lateral leading cutting edges are sharpened and the blade fingers are twisted about their longitudinal axes lowering the sharpened cutting edges below the rest of the body of the blade fingers. This blade configuration permits only the lateral leading cutting edges of the rotating cutting blade member to contact the surface of the bed knife making contact with the bed knife teeth while rising along the lateral cutting edges of the bed knife teeth and subsequently falling off of the lateral trailing edges of these teeth. The flat central portion of the cutting blade, as it is forced by the spring pressure imparted by the bowed cutting blade fingers into contact with the flat washer above it, stabilizes the rotating cutting blade member so that the cutting edges of the bowed twisted blade fingers maintain a proper shearing attitude.
The bed knife of this improved rotary shear is also specific in it's configuration. In the preferred embodiment there are eleven projecting teeth evenly spaced about the periphery of the bed knife. An odd number of identically shaped pointed toothed projections evenly spaced about 360 degrees of the circumference of the bed knife will result in the tip of any given tooth and a straight line projected from that tip across the center of the circular bed knife to a point diametrically opposite on the circumference encompassing the tips of the toothed projections will put that point equidistance between the tips of the toothed projections on either side of that point. It is also possible to use an even number of evenly spaced identical teeth in which case the preferred shape of the rotary cutting blade must be altered to achieve the desired rocking cutting action.
In the preferred embodiment the flat bed knife is four inches in overall diameter and the toothed projections are seven-eights of an inch long. The number of degrees in the angle of a lateral cutting edge of a bed knife tooth from a line drawn from it's tip to the center of the bed knife is determined solely by preference and need not be equal to the angle from said line formed by the lateral trailing edge of the triangular tooth. In the preferred embodiment, for optimum performance, the aforementioned angles are such, in combination with the shape of the rotary cutting blade member, that at the moment that one lateral leading cutting edge of a rotating blade finger starts it's cut at the inner portion of the lateral cutting edge of a given bed knife tooth, the lateral leading cutting edge on the opposite blade finger of the rotating cutting blade member is just beginning to fall off the trailing edge of a tooth on the other side of the bed knife. This opposite cutting edge continues to fall off the lateral trailing edge of this tooth as the lateral leading cutting edge of the blade finger actually shearing is rising along the lateral cutting edge of the bed knife tooth it is in contact with. At the completion of this cut the opposite blade finger has just completed falling off of the trailing edge of the bed knife tooth with which it is in contact. This action repeats itself 22 times upon the 11 bed knife teeth of the preferred embodiment in the 360 degrees of rotation of the cutting blade member and the result is a rocking see-saw like cutting motion for highly efficient and energy conserving cutting action.
In accord with a further specific improvement the diameter of the rotary hub drive member is such that the periphery of the rotary hub drive member overlaps a portion of the toothed projections of the bed knife. This configuration assures that all grass and the like entering into the spaces between the bed knife teeth is within the cutting area of the grass shear and there are no exposed surfaces of the bed knife where grass and the like can accumulate and affect the efficiency of the tool.
It is also a specific function of the rotary hub drive member to prohibit grass and the like from entering into the housing of the grass shear and, of equal importance, a portion of the undersurface of the rotary hub rides in juxtaposition and parellel to the flat upper surface of the bed knife thus preventing grass and other growth from entering the area betweem these two surfaces and inhibiting the rotary motion of the grass shear.
It is accordingly an object of the present invention to provide a new and improved grass shear utilizing a stationary bed knife, a rotary hub drive member and a rotary cutting blade assembly.
It is a further object of the present invention to provide an improved grass shear having a stationary bed knife and a rotary blade assembly and wherein improved cutting action is achieved by means of specific improvements affecting the cooperation between the rotary cutting edges and the teeth of the stationary bed knife.
A further object of this invention is the provision of an improved grass shear wherein loading of the energy source for the shear is minimized by the blade action and the cooperation of the rotary hub drive member and the bed knife.
Further important objects of the present invention are the provision of an improved grass shear assembly which embodies relatively low manufacturing and maintenance costs and yet is characterized by efficient operation and long life.
Additional objects of the present invention will become apparent from a consideration of the detailed description taken in conjunction with the attached drawings.